French flanker Anthony Jelonch will miss the upcoming Six Nations with a right knee injury, a source close to the Toulouse star told AFP on Monday.
The 27-year-old suffered the injury in his Top 14 club's 31-19 win over Bath in the Champions Cup on Sunday.
"He felt his knee move. We'll wait for the tests," Toulouse coach Ugo Mola said after the match.
"We preferred not to take any risks. I dare to hope that it's nothing more than a minor issue."
However, a rupture of the cruciate ligaments is now feared.
The 29-times capped back rower had returned to figure for the hosts in the 2023 World Cup after recovering from a ruptured anterior cruciate ligament to his left knee in last season's Six Nations victory over Scotland.
The former Castres player had a six-month race against the clock to participate in the World Cup, and he started the second match on September 14 against Uruguay.
France face Ireland in their Six Nations opener in Marseille on February 2.